1. Field of the Invention
The invention in general relates to flaw detection, and more particularly to flaw detection using the eddy current principle.
2. Description of the Prior Art
If a current carrying coil is brought near a metallic member, localized eddy currents are produced in the member. If a pick-up or sensing coil is placed adjacent the member in the vicinity of the eddy currents a corresponding voltage will be produced by the pick-up coil indicative of these currents. This principle is utilized in a technique to detect surface cracks and shallow defects in the metal object since the pick-up coil will produce a first signal in the absence of any flaws and a second and different signal in the presence of these flaws.
The coil arrangement is mechanically scanned over the member while it is off-line and by recording the pick-up coil's output signal, as well as the location of the coil arrangement relative to the member under test, an indication may be obtained of the presence and location of any flaws.
With such an arrangement, because of skin effects, the depth at which flaws may be detected is a function of the frequency of the excitation current. For example, to detect a crack in a typical steel member one inch (2.54 cm) from the surface would require an excitation current frequency of approximately 1 Hertz (Hz). The detection of a signal at this low frequency is relatively difficult and accordingly, frequencies in the order of 60 Hz and higher are generally used with such apparatus. 60 Hz, however, corresponds to a maximum detection depth of approximately 1/8 inch (0.317 cm) which may be detected with facility and hence, cracks at depths below this value may go undetected.
The present invention utilizes eddy current techniques for the detection of defects in rotating members in an arrangement which can detect cracks (or other anomalies) at depths exceeding those detectable with equal facility by the mechanically scanned off-line arrangement.